The paper reports a unique study of contracting for infectious disease control in the English National Health Service internal market. New-institutional economic and socio-legal theories are used to predict that it will not be possible to make contracts for the control of infectious disease which are complete with respect to either the allocation of financial risk or the specification of quality standards. Socio-legal theory predicts that in some circumstances informal relationships (known as networks or relational contracts) can evolve to compensate for some of the deficiencies of incomplete contracts. NHS policy makers are shown to have expected and continue to expect that contracts can be complete. The study comprised five in-depth case studies of contracting in different local areas in England and a national survey of all infection control professionals. The results confirm the theoretical predictions. Contracts were not complete, and some of the elements of relational contracts were present. These elements did not, however, fully compensate for the lack of clear accountability caused by contractual incompleteness.